Global variable 0-initialized penalty

Question

This is a very simple question:

Does 0-initializing global and static variables have any performance penalty (albeit very small) at runtime?

Answer 1

No, since the C++ (and C) standard says that all global/static variables that are not initialized explicitly by the programmer, must be initialized to zero. Such variables are placed in a special segment called .bss. They are initialized to zero before main() is called.

If you initialize your global/static explicitly, but to the value 0, the compiler is smart enough to realize this and still put it in the bss segment.

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You can test this for yourself with an example like this:

#include <stdio.h>

static int uninit;
static int init_zero=0;
static int init_one=1;

int main (void)
{
  printf("%p\n", &uninit);
  printf("%p\n", &init_zero);
  printf("%p\n", &init_one);

  return 0;
}

In this example, the uninit and init_zero variables will end up at adjacent memory addresses (likely 4 bytes away from each other), since they are both in the .bss segment. But the init_one variable will end up somewhere else entirely, because it is allocated in the .data segment.